Since the organic EL devices are self-luminescent devices, they are bright, excellent in visibility, and capable of giving clear display as compared to liquid crystal devices, and studies thereon have actively been conducted.
In 1987, C. W. Tang et al. of Eastman Kodak Co. have turned an organic EL device using an organic material into practical utilization by developing a multilayered structure device wherein various functions are respectively distributed to materials. They stack a fluorescent material capable of transporting electrons and an organic substance capable of transporting holes, and injected both of the charges into the fluorescent material layer to emit a light, thereby achieving a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (see Patent Document 1 and Patent Document 2, for example).
Patent Document 1: JP-A-8-48656
Patent Document 2: Japanese Patent No. 3194657
Many improvements have been made up to the present for putting organic EL devices into practical use, and high efficiency and durability have been achieved by an electroluminescence device wherein an anode, a hole injection layer, a hole transport layer, an emitting layer, an electron transport layer, an electron injection layer, and a cathode are provided in this order on a substrate, thereby further segmentalizing various roles (see Non-Patent Document 1, for example).
Non-Patent Document 1: Preprints for 9th Workshop of Japan Applied Physics, pages 55 to 61 (2001)
Also, there has been an attempt for utilizing a triplet exciton in the aim of a further improvement in luminescent efficiency, and use of phosphorescent materials has been studied (see Non-Patent Document 2, for example).
Non-Patent Document 2: Preprints for 9th Workshop of Japan Applied Physics, pages 23 to 31 (2001)
The emitting layer may be produced by doping a compound having carrier transport property, which is generally called a host material, with a fluorescent material or phosphorescent material. As disclosed in the above-mentioned workshop preprints, selection of an organic material in an organic EL device greatly influences on various properties of the device, such as efficiency and durability.
In organic EL devices, luminescence is obtained when charges injected from the electrodes are re-combined in the emitting layer. Since the transfer rate of holes is higher than that of the electrons, a problem of reduction in efficiency caused by a part of holes passing through the emitting layer arises. Therefore, an electron transport material enabling easy electron transfer has been demanded.
Although tris(8-hydroxyquinoline)aluminum (hereinafter abbreviated as Alq) which is a typical luminous material is also generally used as an electron transport material, the electron mobility thereof is considered to be low. Therefore, as a material having a high electron mobility, 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (hereinafter abbreviated as PBD) and the like have been proposed (see Non-Patent Document 3, for example).
Non-Patent Document 3: Jpn. J. Appl. Phys., 27, L269 (1988)
However, it has been pointed out that PBD has poor stability in its thin film state, such as readily undergoing crystallization, and no satisfactory device property has been achieved.